Polymerization inhibitor



United States Patent POLYMERIZATION INHIBITOR No Drawing. Application December 23, 1957 Serial No. 704,265

4 Claims. (Cl. 183-115) This invention relates to the inhibition of polymerization. More particularly, it relates to the inhibition of undesirable polymer growth in processing equipment during the purification of acetylene by absorption techniques.

When acetylene is produced from hydrocarbons, e.g., by the well known electric arc process, by thermal decomposition or dissociation, or by partial oxidation, the acetylene-containing gaseous mixture produced generally contain so'ealled poly-formers, that is, compounds of the nature of biacetylene, for example, which polymerize to solid materials under certain conditions of temperature, pressure, etc. The purification of such an acetylenic gas mixture is usually accomplished by the selective action of suitable solvents. A number and variety of solvents have been proposed among which may be mentioned, for example, water, acetone, dimethylformamide, butyrolactone, N-methyl pyrrolidone, triethyl phosphate, and diethyl carbonate. The normal procedure for separation and recovery of acetylenefrom gas mixtures by means of absorption in a preferential liquid solvent comprises absorbing the acetylene in the solvent in an absorption tower, removing those gases which are slightly soluble in the solvent in a stripping tower and then subjecting the acetylene solution to desorption which is efiiected by means of increased temperatures, decreased pressures, or a combination of both. However, in these conventional methods of absorption and stripping the gas mixture is exposed to conditions under which the poly-formers mentioned above polymerize and provision must be made in all such processes for periodic removal of the polymeric substances to prevent their excessive build-up in the solvent in the absorption and stripping steps. This requires additional, specialized filtration equipment as well as added process steps which are costly with respect to both time and capital expense. Frequently, too, the polymeric material collects in the less accessible parts of the system, fouling up equipment lines, columns and the like. Usually, too, the polymeric material as formed is of such a small particle size that it plugs the filter used for separating it, so that only a small amount can be filtered in each operation and an inordinate length of time is required to wring the filter cake dry.

Methods have been proposed for the removal of the polyformers from the gas stream but these are cumbersome and expensive since they require additional process steps prior to the usual solvent purification operation which utilize special equipment and significant quantities of special chemicals for treating purposes.

The difiiculties in the prior art processes have been obviated to some extent by the discovery that the polymerization of biacetylene, one of the poly-formers in the acetylene-containing gas, can be inhibited by addition of methylene blue to the solvent in which it is contained. Recently, it has also been determined that a minor amount of certain compounds, namely hydrazine or hydrazine hydrate, nitrophenols, and quinone among others, incorporated in the solvent effectively inhibits the formation of polymers in an acetylene purification system wherein the solvent is employed for the selective absorption of acetylene from an acetylene-containing gas stream and thereafter the resulting solution is stripped to recover the acetylene. It has now been discovered, surprisingly enough, that the combination of certain of these inhibitors with methylene blue has an effect far and above what could be expected from their combined individual action, i.e., that some synergism occurs when these inhibitors are combined which results in a significant reduction in the amount of polymer formed in the acetylene purification system under consideration.

It is an object of the present invention, therefore, to provide an improved method whereby undesirable polymer growth during the solvent purification of acetylene is inhibited. Other objects and advantages of the invention will be apparent from the following description and the appended claims.

According to the invention, the formation of polymers in an acetylene purification system wherein a solvent is employed for the selective absorption of acetylene from an acetylene-containing gas stream derived from hydrocarbons is eifectively inhibited by incorporation in said solvent of a minor amount of a mixture of methylene blue and hydrazine.

The invention is illustrated in the following examples which, however, are not to be construed as limiting it in any manner.

Example I The testing apparatus consisted of a series of Pyrex gas-washing bottles (500-ml. capacity) connected in parallel to gas-inlet and outlet manifolds. The distributing outlets on the gas-inlet manifold all contained steel needle valves and rotameters for reglulating and measuring the gas flow. Three sets of tests were conducted. In each set, one bottle was filled with the N-methyl pyrrolidone solvent alone while the second was filled with N-methyl pyrrolidone to which hadbeen added, respectively, a small quantity of methylene blue,hydrazine, and a 1:1 mixture of methylene blue and hydrazine. The solvent was carefully weighed into each bottle so that the same amount, 400 g. (approximately 400 ml.), was used in each one. An acetylene-containing gas stream was bubbled through the liquid in the bottles at a fixed rate of about 600 ml. per min. for a period of approximately 48 hr. The gas employed was the efiiuent stream produced by the oxidation of methane. It was tapped from a process line in an acetylene plane at a point downstream from the furnace or reactor after the gas had been quenched and filtered to remove any soot present. This gas was then successively passed through a compressor, a pressure regulator and a chilled surge container prior to admittance into the gas inlet manifold.

After the bubblers had been in operation for 48 hrs;, the bottles were disconnected and the solutions were weighed. The contents were then transferred to clean glass bottles and analyzed for water and polymer content. Polymer determinations were made as follows: Ten-ml. samples of the solutions were pipetted into 65-ml. ignition tubes. The solutions were frozen solid by immersingthe tubes in a trichloroethylene-Dry Ice bath and the tubes were then sealed with an oxygen-natural gas torch. After allowing them to warm up to room temperature, the tubes were placed in an oven maintained at C. for 10 hours. At the end of this time, the tubes were removed, allowed to cool to room temperature, then further cooled to solidify the contents again by freezing, and opened. The opened tubes were allowed to warm up to room temperature after which the contents of each were poured into separate Erlenmeyer flasks and 15 ml. of 1 N H 50 was added to each flask. The flasks were heated on a hot plate until the mixtures therein were brought to a near boil. The contents of each flask were then filtered on separate tared Gooch crucibles, the flasks being rinsed with hot water and the washings being poured onto the filters. The crucibles were dried in a vacuum oven at 100 C. for I hr., cooled in a desiccator and weighed to determine the amount of polymer present in each sample.

Results of the tests are presented in Table I.

TABLE I Sample No X-1 X-3 VIII-1 VIII-5 XII-1 XII-14 d None--- None.-- Methylene Inhibitor Added None Him? blue and hydrazine. Amt. of Ir;hibltor (Wt. Percent 01' do- 0.5 -do 0 -.do 0.4 (0.2 each).

Solvent Wt. Gairgigbggollvent (g.) 11.3 13.9 16.7 11.5"--- 13.5.

r t o vent: Pa I riitial: 0.63 0.63 0.62-.- 0.90"--. 0.90.

Final 2.10"-.- 2.00"--- 3.27---" 2.00"--- 2.06. Percent Polymer- 1.31 0.58 1.11 1 35--. 0.26. Percent Polymer in Inhibited Solu- 0.44- 0.19.

thin/Percent Polymer in Uninhiblted Solution.

These data convincingly demonstrate that a mixture Example I! A gas mixture obtained by the partial combustion of methane with oxygen in a suitable furnace containing the following constituents in mol percent C,H, 7.5 CH 4.0 CO 26.7 C0, 3 .0 H, 56.0 N, 1 .3 Argon, biacetylene, acetylene polymers. polyformers, etc. 1.4

is compressed and fed under pressure into the middle of an absorption tower. This is a tower containing trays, pans, or ferrules of any suitable type. The gaseous mixture ascends into the upper portion of the tower coming into contact therein with descending liquid N-methyl pyrrolidone containing in solution about 0.5% by weight of a 1:1 mixture of methylene blue and hydrazine admitted at the top of the tower. As the two streams of ascending vapor and descending liquid come into contact, the acetylene in the gas dissolves in the N-methyl pyrrolidone so that by the time the liquid solvent reaches the bottom of the absorber it is saturated with acetylene. It contains also small amounts of the other volatile constituents which are slightly soluble such as carbon dioxide and ethylene. An overhead gas essentially free of acetylene is withdrawn from the top of the absorber.

The solution of acetylene from the bottom of the absorber is introduced into a desorber where the pressure of the gas in contact with the solution is reduced with the result that the major portion of the less soluble nonacetylenic gases is flashed off. These less-soluble gases are returned to the scrubber for recovery of any acetylene contained in them. The acetylene-saturated solution of N-methyl pyrrolidone leaving the desorber passes through a heater wherein its temperature is raised and is then conducted into a series of stages or zones wherein the solvent is subjected to progressively lower pressures and mediate section of the desorber. The liquid solvent from the bottom of the stage of lowest pressure substantially completely freed of acetylene is pumped to a solvent dcsorber where it is freed of remaining impurities and recycled to the absorption tower.

In this system where a mixture of methylene blue and hydrazine is incorporated in the solvent, polymerization of the unsaturated polymerizable components in the gas being treated is held to a minimum. No appreciable amounts of polymer are deposited in the solvent or in the equipment thus obviating the usual processing difficulties arising from plugged pipes, trays. etc.

The eifectiveness of the inhibiting mixture is not restricted to those conditions given in the examples. Usually, a 1:1 ratio of methylene blue to hydrazine is employed although other ratios may be used. In general, only minor quantities of the inhibiting mixture are required although the amount used depends somewhat on the amount of poly-formers present in the gas stream and the degree of inhibition desired. Concentrations of the mixture in the range from about 0.01 to about 5% by weight of the solvent are suitable, with concentrations in the range from about 0.1 to about 1% being preferred. The stabilizing or inhibiting mixture may, of course, be employed in as great a concentration as desired.

The inhibiting mixture of the invention is not restricted to use only with N-methyl pyrrolidone as set out in the examples above. It may be used to inhibit polymeriza tion in other like organic solvents such as bu yl'olactone, for example. In any solvent system where a selective solvent is used for the recovery of acetylene from a gas stream containing it in combination with other gases including poly-formers by absorption in the solvent and subsequent stripping therefrom, polymerization of such poly-formers is effectively inhibited by the presence of a minor amount of a mixture of methylene blue and hydrazine in the solvent. To inhibit polymer formation during the absorption-stripping operation, the inhibiting mixture can be added directly to the solvent feed stream, or it can be introduced as a solution of the desired cons centration at some point in the absorption system. The inhibiting power of the mixture of methylene blue and hydrazine is not affected by continual re-use; hence, re= circulation of the solvent may be safely practiced with the inhibiting effect of the additive mixture of the inven= tion suffering no significant decrease in effectiveness.

What is claimed is:

n a pr s for h recovery of a e ylene hem an y n on inin ga s r m d r e it hydro r n wh r i th e y ne-con aini ea, stream. is contacted with a solvent for selective absorption of ty n ther fr m and s id solut on is th r ait rf s r pped,

to recover acetylene therefrom, the improvement which comprises incorporating in the solvent employed a 1:1 mixture of methylene blue and hydrazine, said mixture being present in an amount in the range from about 0.01% to about 5% by weight of the solvent, said solvent being non-reactive with a 1:1 mixture of methylene blue and hydrazine under the conditions employed.

2. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with a solvent for selective absorption of acetylene therefrom and said solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating in the solvent employed a 1:1 mixture of methylene blue and hydrazine, said mixture being present in an amount in the range from about 0.1% to about 1% by weight of the solvent, said solvent being non-reactive with a 1:1 mixture of methylene blue andhydrazine under the conditions employed.

3. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating in said N-methyl pyrrolidone solvent a 1:1 mixture of methylene blue and hydrazine, said mixture being present in an amount in the range from about 0.01 to about 5% by weight of the solvent.

4. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein the acetylene-containing gas stream is contacted with N-methyl pyrrolidone for selective absorption of acetylene therefrom and said N-methyl pyrrolidone solution is thereafter stripped to recover acetylene therefrom, the improvement which comprises incorporating in said N-methyl pyrrolidone solvent a 1:1 mixture of methylene blue and hydrazine, said mixture being present in an amount in the range from about 0.1 to about 1% by weight of the solvent.

References Cited in the file of this patent UNITED STATES PATENTS 2,664,997 Eck Jan. 5, 1954 

1. IN A PROCESS FOR THE RECOVERY OF ACETYLENE FROM AN ACETYLENE-CONTAINING GAS STREAM DERIVED FROM HYDROCARBONS WHEREIN THE ACETYLENE-CONTAINING GAS STREAM IS CONTACTED WITH A SOLVENT FOR SELECTIVE ABSORPTION OF ACETYLENE THEREFROM AND SAID SOLUTION IS THEREAFTER STRIPPED TO RECOVER ACETYLENE THEREFROM, THE IMPROVEMENT WHICH COMPRISES INCORPORATING IN THE SOLVENT EMPLOYED A 1:1 MIXTURE OF METHYLENE BLUE AND HYDRAZINE, SAID MIXTURE BEING PRESENT IN AN AMOUNT IN THE RANGE FROM ABOUT 0.01% TO ABOUT 5% BY WEIGHT OF THE SOLVENT, SAID SOLVENT BEING NON-REACTIVE WITH A 1:1 MIXTURE OF METHYLENE BLUE AND HYDRAZINE UNDER THE CONDITIONS EMPLOYED. 